Weak gravitational lensing and cluster mass estimates

Hierarchical theories of structure formation predict that clusters of galax ies should be embedded in a weblike structure, with filaments emanating fro m them to large distances. The amount of mass contained within such filamen ts near a cluster can be comparable to the collapsed mass of the cluster it self. Diffuse infalling material also contains a large amount of mass. Both of these components can contribute to the cluster weak lensing signal. Thi s "projection bias" is maximized if a filament lies close to the line of si ght to a cluster. Using large-scale numerical simulations of structure form ation in a Lambda-dominated cold dark matter model, we show that the projec ted mass typically exceeds the actual mass by several tens of percent. This effect is significant for attempts to estimate cluster masses through weak lensing observations and will affect weak lensing surveys aimed at constru cting the cluster mass function.